Process and apparatus for crystallization



Nov. S', 1925. n

H. HOWARD PROCSS AND, APPARATUS FOR CRYSTALLIZATION Filed July 5. 1923 Patented Nov. 3, 1925.

' UNITED STATES PATENT OFFICE.

HENRY HOWARD, 0F CLEVELAND, OHIO, ASSIGNOR TO THE GRASSELLI CHEMICAL COMPANY, OF CLEVELAND, OHIO, A CORPORATION OF OHIO.

PROCESS AND APPARATUS FOR CRYSTALLIZATION.

Application led J'uly 5, 1923. Serial No. 649,715.

To all 'w71 om t may concern:

Be it known that I, HENRY HOWARD, a citizen of the United States, residing at Cleveland, in the county of Cuyahoga and State of Ohio, have invented certain new and useful Improvements in Processes and Apparatus for Crystallization, of which the following is a speciication.

This invention relates to process and apparatus for crystallization, and more particularly to the direct production of crystal products of substantially uniform predetermined sizes.

It has been found heretofore that crystallization may be advantageously carried out in solutions in motion and seve-ral forms of apparatus, among which may7 be mentioned apparatus provided with stirrers for agitating the crystallizing solution, have been designed for this purpose. It has also been proposed to cause crystallization to take place iu the foaming or boiling upper layer of an upwardly flowing current of the crystallizing solution. These processes possess certain advantages, particularly with reference to the production of relatively pure fine-grained crystals and rapid crystallization.

The' present invention relates to certain improvements in said prior processes and to apparatus for the carrying out of the iinproved process in which the valuable cliaiactcristics of the prior processes resulting from the crystallization in a solution in motion are combined with a grading or sortingaction accomplished hy the movement of the crystallizing solution, whereby crystal products of substantially uniform and predetermined size of crystal grain and of high 'curity are produced directly at an economi cal rate.

The invention consists generally in causing crystallization or crystal growth to take place in an upwardly flowing current of the crystallizing solution which is so handled that crystals of the desired size formed in the solution are permitted to sink down through the solution to a collecting s ace, while smaller crystals are maintain y in suspension in the solution until they have either grown to the desired size or are dis'- solvedrin the crystallizing solution.

. Theiiirocess will be described more in delthe solution delivery pipe 6.

tail in connection with apparatus suitable for carrying out the process and embodying my invention, several forms of which are illustrated in the accompanying'drawings in which Fig. 1 is a vertical elevation of complete apparatus in accordance with my invention, an

Figs. 2, 3 and 4 are vertical elevations of modified forms of crystallizing vessels.

Referring to Fig. 1,v 1 is the crystallizing vessel which, as illustrated, comprises three cylindrical sections 2, 3 and 4. of progressively increasing diameters in an upward direction connected by sloping walls. The lower section 2 communicates with the trap 5 which has a. relatively large cross-sectional area as compared with the section 2. The pipe 6 enters the side of the trap 5 for the supply of crystallizing solution, and the pipe 7 connects with the lowermost region of the trap 5 foi the dischargey of crystalsvcollecting therein. The crystallizing solution after passing up through the crystallizing vessel 1 is delivered through the overflow 8 to waste or to purifying apparatus not illustrated or as is illustrated directly to the receptacle 9 where it is reconcentrated, for instance, by the addition of fresh concentrated solution delivered thereto as indicat ed at 10. the i-econcentrated solution f1owing from the vessel 9 as indicated at l1 to 12 is a pipe for the introduction into the crystallizing vessel 1 of fine crystals or supersatiirated solut ion for forming fine crystals to be grown, an-d 13 is a heating coil for melting fine crystals as will be explained hereinafter.

The form of apparatus illustrated in Fig. 1 may be varied extensively withoutl de-V parture from the principles involved. For instance, instead of the three sections 2,v 3 and 4 the crystallizing vessel may comprise a greater or less number of-sections of pro illustrated as spherical, may be of any o er suitable shapehfor instance, rectan lar or conical. The vpipe 6 of course shoul empty into the trap 5 such a way as not to inter-` a screen or perforated baille to avoid' the` pro ,liquid currents in the tra The crystal d1svessel.

fere with the settling of c stale/therein. For this purpose it may be a vantageous to have it deliver liquid into the trap through plate or against a notion of strong charge ipe 7 is designe to conve' a slurry of the nished crysta s with a s cient current of the c r stalllzing solutionl to carry 10 them along. T e pipe 7 may be replaced by any other suitable means for the discharge of crystals such as a simple outlet or drain cock, a boot and bucket conveyor or the like.

The crystallizing vessel may take otherv forms as illustrated in Flgs. 2, 3 and 4.

Fig. 2 illustrates a simple conical vessel. It has been found in practice that the angle between the lines of intersection of the walls of the cone with a plane through the aXis of/the cone should be lessV than 60 in order to secure an efficient sorting of the crystals in this type of crystallizing vessel. A conical vessel one inch in diameter at the bottom,

two inches'in diameter at the top and four fect high gives satisfactoryl results. The

conical vessel illustrated in Fig. 2 may advantageously be flared toward the top, that is, above the effective sortingr zone. As 1s illustrated the heating coil is situated slight- ,30 ly below the overflow and the pipe for the introduction of supersaturated brine or seed crystals farther down in the 'crystallizing If the vessel is flared at the upper end as suggested, the heating coil will be placed within the ,flared portion and the pipe for the introduction of supersaturated brine or seed crystals at about the beginning ofthe fla-red portion. .j

Fig. 3 illustrates still another modification 40 of the crystallizing vessel, comprising a lower conical section 14, the angle of whichis less than 60 for the reasons stated above, and an upper communicating conlc'al section 15, the walls of Which diverge upwardly at 4 5 a somewhat greater angle.

The upper section 15 provides for the slow movement of the crystallizing solution therein and thus permits the sedimentation or melting or both'of the tine 'crystals in suspension and prevents 'their passage through the overflow.

As illustrated, the heating coil is located in ythe upper section 15, and the pipe for supplying seed crystals enters the crystal- 112mg vessel near the upper endiof the lower section 14.

Fig. 4 illustrates still another form of crystallizing vessel in the form Vof a single cylindrical vessel or pipey of relatively small diameter as compared with its height l having a conical bottom of small angle,

preferably lessthan 60, which insures an eflicient sorting or separation of the coarse from the fine crystals.

' It is to be understood that the forms of apparatus illustrated are not regarded as representin every conceivable formof apparatus em odying the principles of the ini vention. l

The operation :of the process infthe apparatus illustrated in Fig.`1 now tobe described is analogous to the operation of the process in the apparatus illustrated in Figs.

2, 3 and 4 and will suflice to enable those' skilled in the art to understand the invention.

A solution of a material tobe crystallized is supplied to the vessel 9 where it is brought to theA desired concentration and tempera-lY ture` for instance, by distillation or by;

heating, if necessary. and the 'addition of fresh concentrated or'rsupersaturated soin-5'* tion as indicatedat 10, and the resultingr solution is delivered at 11 to the pipe 6 flowing 'therethrough into the trap 5. From4 the 1 trap 5 a part 4of the solution supplied thereto Hows through the pipe l7 carryingicrys-A tals from the trap to suitable lmeans not illustrated for' washingl and drying them,

to be grown nay be supplied to the crystallizing vesselin a variety of ways, for nstance, primary. crystallization in the crystallizing V.solution 1 may be caused to take place by suflicient cooling ofthe crystal- 1 lizing solution in the crystallizingyvessel.

This procedure, however, is not preferred and is very difficult to control. A cooling of the whole body of the solution' in the crystallizing vessel to a sulficientextent'to cause primary' crystallization in the case of many materials and unless the cooling is very gradual and precisely clontrolled is apt to result in they formation o'f too great a number of 'fine c1ystals,`even to the extentl of forming a crystal slush which interferes; with or even renders impossible the desiredgrowth of large crystals and efficient separation of the coarse crystals from the fine crystal slush. Another lmethod of supply ing fine crystals to be grown which offers more satisfactory possibilities is by local cooling of the flowing solution` for instance, by means of ,a small refrigerating-coil. not illustrated, situated in the section 3 of the crystallizing vessel. The preferred procedures areto introduce into thecrystallizing vessel through the Dine 12 a slurry of or containing fine crystals formed elsewhere.I

or to introducemthrough the `pipe 12 a regulated small of tion of the 'material f ffbe crystallized or a r:su-persaturated solunuages solution of the material of suiiicient concentration and at a temperature sufficiently above that of the crystallizing solution that take place.v

spontaneous primary crystallization .will

,4 The relative sizes and len ths of the sections 2, 3 and 4, the rate of ow of the crystallizing solution through the sections, the l temperature and concentration of-thef'solution supplied to the crystallizing vessel and the rate of cooling of the solution in the crystallizing vessel to induce crystal growth' erature atwhich the solution is supplied.

or instance, .if slow cooling is desired, as is usually the case, the solution -is supplied -at only a little above atmospheric temperature, while if rapid ,cooling is desired a larger difference betweerr the temperature; of

the solution supplyl and atmospheric tem-- perature is maintained. It is of course preferred from an economic standpoint to use apparatus in which the cooling fromthe walls of the crystallizing vessel to the surrounding atmosphere is suiicient, but u'ndei some circumstances it-may be desirableto eitherefrigerate the walls of the vessel or to insulatel the walls to prevent too rapid cooling or even to subject the walls to a temperature above atmospheric. to prevent too rapid a cooling of the crystallizing. solu-v tion. It is noted in this connection that the cooling of the solution in the' crystallizing vessel should not be so rapid as to produce any substantial vprimary crystallization, excepting as suggested above for the production of seed crystals. The rate of flow in.

the crystallizing vessel, and particularly A through the section 2, is regulated so that only crystals of the desired maximum size are able to sink downwardly therethrough and fallinto the trap 5. In the section 3 the iiow of the solution is less rapid 'and it is in this section that the principal crystal growth takes place. The iiow of solution in rsection 3 however is maintained suiiiciently rapid to hold fine crystals in suspension but sufficiently slow to permit crystals which have reachedthe desired size to sink. Section 4 serves to catch and hold back from vthe overflow 8 the fine crystals which are carried upward by the fiowing solution. Some crystal growth takes place in the sec. tion 4 and the resulting larger crystals drop into section 3 Where they grow further, and the operation of the section 4 may be suf.-

licient for this purpose 'or other larger sec-.

tions may be placed on .top of the section 4 as suggested. The preferred procedure,

.howeve-r, vin order to avoid an undesirable multiplication of sections and also toavoid the? passage of fine crystals out of the crys# tallizing vessel with the overflowingv so u.- tion is to heat the solution ,by means of the heating coil 13 just before it reaches the overow. any small .crystals reaching this point in the crystallizing vessel. This procedure also has the advantage that it -prevents* the overchar'ing of the crystallizing vessel with smal crystals'in 'case any primary crys tallization takes place, and further, in that,

it raises the temperature of the crystallizing solution entirely or in part to the desired.

point for recirculation.` The procedure has This serves to dissolve or m'elt still another advantage when the process is' `used as-it maybe to produce a crystal-prod?. uct 'o'f-umform size from a-mixture'of crystals of different sizes obtained froml some outside'source. In this case vitmay be assumed that' there is no solution of thematerial at hand andthat nothing but solvent and a mlxture of crystals` is supplied tothe4 crystallizingapparatus. The larger crystals in the mixture must be grown at the exaccomplished by melting the small crystals inthe section tand then returning the resulting .solution tothe bottom of the cryspense of the smallerones.v This of course is l tallizing vessel where in its passage upward it serves as the-medium for the growth of the 'larger crystals suspended therein. Even when freshsolution of the material is supplied from an outside source to provide for the growth of crystals introduced into the crystallizing vessel from an outside source the use of the heating coil 13 is ordinarily advantageous, because such a mixture of crystals ordinarily contains more .fine crystals than can be grown to the desired size from the material supplied in the form of fresh solution. v

Thevsolution leaving the crystallizing vessel 1 by wayof overflow 8 may be discharged to waste or conveyed-to some other use or, if impure, it may be purified and delivered to the tank 9 for reconcentration, or if puriication is unnecessary it may pass directly as indicated to the tank 9. In tank 9 the solution is brought to the desired initial l temperature and is reconcentrated to the fdesired extent eitherl byevaporation or by the addition of fresh highly concentrated solution or both and lis then deliveredagain to the crystallizing vessel, s

I haveI described the provision of means and procedure for preventing any fine crystals from leaving the /crystallizing vessel with the overflowing solution. 'Some of the is that theintroduction of fine crystals into fine crystals from the solution outside -of the crystallizing vessel, for Instance, fbyA melting them in the tank 9 or by filtration or sedimentationat -a suitable point inthe circulation of the solution.

'Ifit is desired to obtain more than one size of crystal product from a single apparatus it will be apparent thata trap may be placed at the' base ofl more than one .of the sections of thel crystallizing vessel 1 and crystals collected and withdrawn therefrom, the si'ze of crystals obtained from each section .depending upon the rate of flow of Solutionin such section or upon the crosssectional area ofthat section. This mode of operation and apparatus embodying the described featureare embraced by my invention, although` in practice Iv preferto employ either separate units of apparatus for producing crystal products of different size or to operate the-same 'apparatus at different times with a different current fiow so as to obtain the` `desired,- different sizes of crystals.

It is noted that thecrystallization or growth of the crystals takes place in a free suspension of the crystals bein grownl in a moving solution of the crysta izing material, as a result of whichthe individual crystals of the crystal product are of their natural shape, :being substantially unabraded by contact with other crystalsor the walls f the crystallizing vessel. The free movement of the crystals in the crystallizing' solution also insures an accurate grading, and the maximum rate of growth.

It is noted that my rocess isdesigned primarily and preferab y is carried out upon solutions at below their'boilingtemperatures and not upon boiling solutions or upon solutions which are supersaturated and at substantially boiling temperature,

'In the `appended claimsthe expression supersaturated solution is used to designate not' only solutions which are supersaturated in the true sense, but also "solu,

tions containing solute in quantity greater than that necessary to produce'saturation vat the `temperature of the crystallizing solu-y tion. l

I am'aware of French Patent No. 542,422.`

I claim:

-1. Process of crystalli-zling whichfrcomprises maintaining a suspension of crystals of a material to be crystallized 1in an ascending current of a solution of the material, in-

` ducing the deposition -of material from said solution upon the crystalsl in suspension, and

supplying. crystals to be grown to said asdcending current-of vsolution by introducing a relatively small stream of a supersaturated solution of they-material into` said ascending current of solution.

2. Process as defined in claim y1 in which `the rate of How of the ascending current of solution throughout its entire cross section a predetermined sizein sus ension and to permit crystals of said pre to settle downwardl v l f 3. Process of producing a crystal product of a uniform predetermined size from a mixture of crystals of a material of' diffen.

"ent`sizeslwhich comprises suspending saidi mixture'v of crystals 1n an ascending .current lof a solution of the material of suiicient strength throughout its entire cross section .to maintain crystals of less than'the predetermined size in suspension and to permit crystals of said predetermined size tosink,

eterlnined.v size' v n is suficient to maintaincrystals of less than andr inducing the deposition* of material from said solution' upon said crystals in suspension. c

4. Process .of crystallizing .which comprises: maintainin a .continuous circulation lof -a solution oft e material to be crystal- 'lized upwardly through a crystallizing vessel 4and through a return circuit, maintain# ing crystals,y ofthe material in suspension in said solution within said crystallizing.'

fvessel, inducing the deposition of material from said solution uponsaid crystals in suspension and reconditioning said solution in said return circuit by the addition of fresh concentrated solution thereto.

5. Process of crystallizing as defined in claim 4 in which thelstrength of the current of solution in said crystallizing vessel is sufficient to maintain crystals' of less than a' lpredetermined size insuspension and to permit;I crystals of said predetermined size to sin 6. Process. of crystallizing as defined in claim 4 in which the strength of the current of solution in said crystallizing vessel is sufficient to v'maintain only crystals of 'less than a predetermined size in suspension and in'. which' the solution `issubstantially,freedA of suspended crystals before leaving the crystallizing vessel by local heating thereof.

7. Processk of crystallizing which com-. prises maintaining an ascending current of a solution of the material to be crystallized having zones of progressively decreasing rate of vflow in an upward direction carrying. crystals of said material in suspension, all portions of said current at any givenlevel flowing at substantially equal rates inducing the deposition of material from said so- ',lution upon said suspended crystals, and maintaining a rate of How of said solution through thelowermost of'said zones-sufficient to maintaincrystals of less than a pre,-

determined size in suspe and to vpermit l crystals of said predetermined'size to sink therethrough. 8. Apparatus for the production of crystals of uniform predetermined size compris- I,

ing' acrystallizing vessel enclosing 'a space of progressively increasm cross-sectional area 1n an upward directlon, the average yrate of said increase incro'ss-sectional area,

1 vessel.v

9.A Apparatusas Adefined in clai1n 8 in' which the crystallizing vessel com rlses a plurality of cylindrical sections of iierent A sizes connected by upwardly divergingwalls.

10. Apparatus for crystallizing comprising a vcrystallizing vessel, means for withdrawing liquid from the upper end of said vessel and for delivering the same to the.

lower end of said vessel, means for heating` the solution adjacent the upper end o f sai vessel, means for introducin crystals of the material to be crystallized into` said vessel and means for introducin freshsolutionoi said material intothe so utio'n withdraw from the ,upper end of said vessel. y

11. 'Apparatus for crystallizing as de-`- mother liquor .from the,

-up ermst section.

. ture.

in claiml-in which said-vessel comlprlse's a pluralityof superposed sections of progressively increasing cross-sectional area f 1n arr-upward direction.'

12'. Apparatus for crystallizingfas defined in claim '10 in vwhich, said vessel comprises a plurality of cylindrical sections of progressively' increasing cross-sectional area in an upward' direction.

13. Apparatus ra ity of sections of `progressively increasing crosssectional area in an upward direction, atrap of. relatively large cross-sectional area communicating with the lowermost of I for lcrystallizing compris- 1n a crystalllzmg vessel comprising la plup l saidv sections, means for withdrawing crys-r `tals from the lowermost region of. said trap,

means for delivering solution" of the material to be'crystallized into said trap at-a point above the lowermost region thereof, a

heating'me'ans -in the uppermost section of said vessel, means for withdrawing' solution from said uppermost section and for deliverin? the same .to said means* for delivering so u tion of thematerial to be crystallized into said trap,- and means. for, supplying fresh solution of the material t'o /be crystallized to the solution withdrawn n testimony whereof, I aix from said;

`my signai HENRY HOWARD. 

